Separation of two immiscible liquids into two distinct liquid layers for recovery of each is well known to the art. However, most separation devices rely primarily on large vessels that use gravity and long residence times to achieve the phase separation or formation of distinct layers. Alternatively, forced physical separation of the two liquids is accomplished using complex mechanical devices, such as centrifuges, which also require a large energy input, or using membranes with selective permeability characteristics. With pressing needs for more economical processes that also are more compact to conserve space, a smaller more efficient separation is needed.
The enactment of the US Clean Air Act of 1990 has reached its zenith in North America with the gasoline pool being required to contain less than 10-wppm of sulfur. This means from a practical standpoint that the refinery normally makes a gasoline pool containing less than 5-wppm to allow for pipeline contamination of residue wall “clingage” sulfur from previous shipments and the accuracy of the testing method dictated by the Clean Air Act.
Another consequence of the Clean Air Act of 1990 has been the shutting down of the small inefficient refiners in America going from 330 plus refiners in 1980 to less than 175 refiners in 2007. No new refiners have been built in the past 25 years, but refiner expansions and imports have satisfied the gasoline demand in America.
The existing refiners have also gone to higher severity Fluid Catalytic Cracking Unit operations to reduce the amount of burner fuel while producing additional higher octane gasoline and increased olefin production. These olefins are propane/propylene and butane/isobutane/isobutylene. These are the feedstocks for the next processing step which is an alkylation unit. Some refiners alkylate amylenes (pentene) depending on their economic models.
Most refineries use either an HF (hydrofluoric acid) or a sulfuric acid alkylation unit to alkylate mixed butylenes or mixed propylene's and butylenes. Alkylation is a process where isobutane reacts with the olefin to make a branched chain paraffin. Since sulfur is detrimental to the alkylation process, a caustic treating system is in place at most refineries to extract the easily extracted methyl and ethyl mercaptans and the more difficult propyl mercaptans present in the mixed olefinic liquid petroleum gas (“LPG”) stream.
Typically, liquid-liquid contactors are employed for the caustic treatment and in some cases fiber-film contactors as described in U.S. Pat. Nos. 3,758,404; 3,977,829 and 3,992,156, all of which are incorporated herein by reference. To conserve caustic, a caustic regenerator is almost always employed. A typical process flow scheme for treating LPG involves a first caustic treatment using at least one liquid-liquid contactor to extract the sulfur contaminants, typically mercaptans, from the LPG feed, which generates a “spent” caustic solution that is rich in mercaptan or so called rich caustic, separating the LPG in the contactor, oxidizing the rich caustic to convert mercaptans to disulfides (typically referred to as disulfide oil (“DSO”)) which generates an “oxidized” caustic solution, and then using a gravity separator to separate the DSO from the oxidized caustic solution. In some instances a granular coal bed is used in conjunction with the gravity settling device as a coalescer to further assist in the separation of the DSO from the oxidized caustic. Once the DSO is removed, the regenerated caustic can then be recycled and mixed with fresh make-up caustic and used in the liquid-liquid contactors to treat the LPG feed.
As mentioned, the use of gravity settling devices in prior art processes are plagued by the requirement of long residence times, especially when applied to separating DSO from an oxidized caustic solution. These long residence times negatively impact the economics of the caustic treating process. In addition, the prior art gravity settlers are relatively large pieces of equipment. Likewise, the forced separation devices, such as centrifuges, are complex mechanical devices that require large energy input to operate. Our invention now solves the problems found in prior art separation equipment when two immiscible liquids need separation and particularly when applied to separating DSO from caustic solutions. Our invention utilizes two novel improvements that can be employed separately or in combination. The first involves the use of fiber-film technology, which is typically only found in liquid-liquid contacting applications, and the second involves the use of solvent injection before oxidation of the spent caustic solution. Our process can also use a polishing step after DSO separation to further remove residual DSO from the oxidized caustic solution. Greatly reduced residence times and the reduction in equipment size translate into an extremely economical method of removing sulfur compounds from LPG, and consequently, minimize capital and operating costs. These and other advantages will become evident from the following more detailed description of the invention.